1. Field of the Invention
This invention relates to fingerprint sensing, especially to a capacitive fingerprint sensing apparatus capable of providing good fingerprint sensing performance and high resolution at the same time.
2. Description of the Prior Art
With the developing of technology, the capacitive fingerprint sensing technology can be widely used in various kinds of electronic devices, especially portable electronic devices such as smart phones, notebook PCs and tablet PCs. Because the fingerprint sensing technology has high resolution requirement, the fingerprint sensing chip should have at least resolution of 500 dpi and its unit sensing area should be 50 um*50 um under the IAFIS standard.
Please refer to FIG. 1. As shown in FIG. 1, the conventional fingerprint sensor array is formed by sensing electrodes SE. It is assumed that the X-direction distance and the Y-direction distance between the sensing electrodes SE are Dx and Dy respectively; when the fingerprint sensing driver 12 drives the sensing electrodes SE to perform self-capacitive fingerprint sensing respectively, if the ridge of fingerprint presses on the sensing electrode SE, the sensing electrode SE will sense larger capacitance; if the valley of fingerprint presses on the sensing electrode SE, the sensing electrode SE will sense smaller capacitance. By doing so, the processing module 14 can obtain a self-capacitive fingerprint sensing pattern having a resolution of 1/Dx along the X direction and a resolution of 1/Dy along the Y direction according to the capacitances sensed by the sensing electrodes SE, wherein the unit pixels in the self-capacitive fingerprint sensing pattern will correspond to the sensing gravity center positions P of the sensing electrodes SE respectively. When Dx=Dy=50 μm, the resolution of the self-capacitive fingerprint sensing pattern will be 508 dpi higher than 500 dpi which can meet the requirement of the IAFIS standard.
Although the conventional self-capacitive fingerprint sensing technology can meet the requirement of the IAFIS standard, there will be a protection layer (e.g., sapphire glass) covering on the fingerprint sensor, so that the fingerprint sensor used in the portable electronic apparatus will not have poor fingerprint sensing effect due to the poor flatness caused by prolonged use or friction. However, because the protection layer usually has a thickness (e.g., 100 μm), if the fingerprint sensor array having a resolution of 500 dpi is used under this condition, the capacitance sensed by the fingerprint sensor array will be lower and easily interfered by noise; therefore, the performance of fingerprint identification will become poor. In order to increase the capacitance sensed by the fingerprint sensor, the size of the fingerprint sensor can be increased; however, the resolution will become lower at the same time.
From above, it can be found that the conventional self-capacitive fingerprint sensing technology fails to have good fingerprint sensing performance and high resolution at the same time. It is urgent to overcome this shortcoming.